Round Top Mountain Rhyolite (Texas, USA), a Massive, Unique Y-Bearing-Fluorite-Hosted Heavy Rare Earth Element (HREE) Deposit

JOURNAL OF RARE EARTHS, Vol. 32, No. 1, Jan. 2014, P. 90

Round Top Mountain rhyolite (Texas, USA), a massive, unique Y-bearing-fluorite-hosted heavy rare earth element (HREE) deposit

PINGITORE Nicholas1,*, CLAGUE Juan1, GORSKI Daniel2 (1. Department of Geological Sciences, The University of Texas at El Paso, El Paso, TX 79968-0555, USA; 2. Texas Rare Earth Resources, 539 El Paso St., Si- erra Blanca, TX 79851, USA) Received 16 September 2013; revised 9 November 2013

Abstract: Round Top Mountain in Hudspeth County, west Texas, USA is a surface-exposed rhyolite intrusion enriched in Y and heavy rare earth elements (HREEs), as well as Nb, Ta, Be, Li, F, Sn, Rb, Th, and U. The massive tonnage, estimated at well over 1 billion tons, of the deposit makes it a target for recovery of valuable yttrium and HREEs (YHREEs), and possibly other scarce elements. Because of the extremely fine grain size of the mineralized rhyolite matrix, it has not been clear which minerals host the YHREEs and in what proportions. REE-bearing minerals reported in the deposit included bastnäsite-Ce, Y-bearing fluorite, xenotime-Y, zircon, aeschynite-Ce, a Ca-Th-Pb fluoride, and possibly ancylite-La and cerianite-Ce. Extended X-ray absorption fine structure (EXAFS) indicated that virtually all of the yttrium, a proxy for the HREEs, resided in a coordination in the fluorite-type crystal structure, rather than those in the structures of bastnäsite-Ce and xenotime-Y. The YREE grade of the Round Top deposit was just over 0.05%, with 72% of this consisting of YHREEs. This grade was in the range of the South China ionic clay deposits that supply essentially all of the world’s YHREEs. Because the host Y-bearing fluorite is soluble in dilute sulfuric acid at room temperature, a heap leaching of the deposit appeared feasible, aided by the fact that 90%–95% of the rock consists of unreactive and insoluble feldspars and quartz. The absence of overburden, remarkable consistency of mineralization grade throughout the massive rhyolite, prox- imity (a few km) to a US interstate highway, major rail systems and gas and electricity, temperate climate, and stable political location in the world’s largest economy all enhanced the potential economic appeal of Round Top. Keywords: heavy rare earth elements; yttrium-bearing fluorite; yttrofluorite; heap leaching; rhyolite; Round Top; deposit

The rare earth elements (REEs) are essential compo- neous rock body related to granite, emplaced as a mush- nents of current and emerging 21st century technologies. room-shaped mass between sedimentary rock layers), en- Recent concern about future supplies of all the REEs riched in rare earths elements, particularly the scarce now has narrowed chiefly to the heavy rare earth ele- YHREEs: Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. The rhyolite ments (HREEs). Essentially all the world’s HREEs cur- is also enriched in Be, Ga, Sn, Nb, Ta, U, and Th. The de- rently are sourced from the south China ion-adsorption posit lies some 130 km southeast of El Paso, TX (Fig. 1[1]). clays. The ability of those deposits to maintain and in- Development of a beryllium mine at Round Top in the crease production is uncertain, particularly in light of en- 1980s was later abandoned. The beryllium deposit lies vironmental degradation associated with some mining along part of the lower contact with limestone; the target and extraction operations in the region. was 3×105 tons of 2% BeO mineralization[1]. The prop- We investigated an yttrium and HREE (YHREE) de- erty now is leased from the General Land Office of the posit that has proven to be unique in its mineralogy and State of Texas by Texas Rare Earth Resources, a US impressive in its tonnage. This unconventional deposit corporation. TRER plans to recover the REEs; it is presents the potential to become a significant source for evaluating simultaneous extraction of lithium, uranium, [1] YHREEs in the near future. and beryllium from the rhyolite as well . The Round Top Mountain rhyolite is enormous; the Texas Bureau of Economic Geology estimated the rock 1 Round Top Mountain HREE deposit mass at 1.6 billion tons[2]. The mountain itself is approximately 375 m tall and nearly 2 km in diameter. 1.1 Nature and characteristics of the deposit Measured, indicated, and inferred total REEs (TREEs) Round Top Mountain, at Sierra Blanca, west Texas, exceed 5×105 t[3]. Approximately 72% of the TREEs in USA, is a rhyolite laccolith intrusion (a fine-grained ig- the rhyolite are the scarce YHREEs[3].

Foundation item: Project supported by Research Contracts 26-8211-12 and 26-8211-16 between Texas Rare Earth Resources, Inc. and the University of Texas at El Paso * Corresponding author: PINGITORE Nicholas (E-mail: [email protected]; Tel.: +1-915-747-5754) DOI: 10.1016/S1002-0721(14)60037-5 PINGITORE Nicholas et al., Round Top Mountain rhyolite (Texas, USA), a massive, unique Y-bearing-fluorite- ... 91

Fig. 1 Location of Round Top Mountain deposit

The deposit is exposed as an erosional remnant that containing grains that could be easily beneficiated by can easily be surface-mined, with essentially no cover to such physical means as gravity or magnetic separation be removed (Fig. 2[1]). Although huge in extent, well ex- are scarce. The REE-bearing minerals are not concen- posed, easily accessible by rail and US interstate high- trated in veins within the rhyolite that could be extracted way, and proximal to electricity and natural gas, the min- by selective mining. eralization is both dilute (TREE grade just over 500 ppm) We have been experimenting in the laboratory at and diffuse (no concentrated zones)[2,3]. The ability to bench scale with direct acid leaching to assess heap economically tap this potential world-class resource leaching as a possible route to low-cost YHREE extrac- hinges on extraction from an ore grade an order of mag- tion. Previous studies indicated that very fine grinding nitude (one-tenth) or more lower than those in conven- and froth floatation concentrates the target YHREEs, but tional REE mining operations, which typically are in the this approach requires a large initial capital expenditure % range. It is, however, within the range of the HREE and significant operating complexity and expenses[3]. ion-adsorption clays of southern China[4]. Further, in ad- Successful extraction of the YHREEs at Round Top by dition to their homogeneous distribution (very little heap leach would be a breakthrough in REE mining variation in concentration throughout the rhyolite), the technology. REEs are hosted in a matrix that is extremely fine grained (generally sub-micron in diameter). Potassium 1.2 REE mineralogy at Round Top feldspar, plagioclase, and quartz grains comprise the bulk, The mineralogy of the YREEs and other critical ele- 90%–95% of the volume, of the rhyolite. Large REE- ments is essential to develop an appropriate extraction

Fig. 2 Round Top Mountain, virtually all of which is mineralized rhyolite

92 JOURNAL OF RARE EARTHS, Vol. 32, No. 1, Jan. 2014 protocol. Previous study of the elemental composition of Table 1 HREE and Y average concentrations in Round Top mineral grains by electron microprobe indicates that the Mountain rhyolite REEs are hosted by bastnäsite-Ce, Ce-bearing fluorite, Elements Eu Gd Tb Dy Ho Er Tm Yb LuY Y-bearing fluorite, xenotime-Y, zircon, aeschynite-Ce, a Concentration/(g/t) 0.2 10.6 3.6 31.7 8.0 32.8 7.1 56.5 8.9 221 Ca-Th-Pb fluoride, and possibly ancylite-La and ceri- Note: Data from Ref. [3] anite-Ce[2,3,5,6]. It was suspected, but never demonstrated, that most of the REEs were hosted in various fluorine minerals. The relevant microprobe analyses were limited 2 Materials and methods by grain size: only “large” particles, mostly 5–10 µm in dimension, could be examined. Aside from the feldspar 2.1 Rhyolite samples and quartz phenocrysts, most of the remaining 5%–10% All but one sample were split from a well-mixed >250 of the rock volume comprises grains a micrometer or less kg sample composited from material recovered from a in diameter. These are not suitable for electron probe mi- representative set of reverse circulation exploration drill croanalysis. The fraction of the REEs in each mineral, or holes into the Round Top Mountain rhyolite. This mate- in unidentified phases too small to analyze, was not, and rial assayed at 211 g/t yttrium. Table 2 lists two samples probably could not be estimated. Optical or microprobe that were sieved to the specified grain size, and a third point counting of grains is the conventional way to quan- sample that was a small block cut from a large grain re- tify the fraction of an element in different phases. This is covered from the drill debris. The remaining 5 samples not possible due to both the low concentration (for Yb at were size-sorted granular rhyolite that was leached for 50 ppm, one searches through a million grains in an at- various periods and temperatures in 8.3 vol.% sulfuric tempt to encounter a single grain of fluorite that is, say, acid. The amount of the initial yttrium remaining in the 2% ytterbium) and minuscule grain size (analogous to samples, relative to the respective untreated material “invisible” gold) of the critical elements in the bulk rock. (first 2 samples), is listed. All samples, except for the Lacking firm knowledge of the mineralogy of the REEs, block, were subsequently reduced in a ball mill to <10 development of an extraction process is forced to be μm grain size, and pressed into a standard 25-mm X-ray based on an extensive and expensive program of trial and fluorescence puck with the addition of a cellulose binder error. and paraffin sealant. The % yttrium remaining (percentage) in the leached solids was determined by X-ray fluo- 1.3 Research goal rescence analysis of the respective untreated and treated To assist in evaluating heap leaching, our goal was to samples, using a PANalytical Epsilon5 energy dispersive determine which mineral(s) host the highly prized spectrometer with a germanium solid-state detector. YHREEs, and in what proportions. For this task we chose synchrotron-based X-ray absorption spectroscopy 2.2 X-ray absorption spectroscopy technique (XAS) experiments, which provide the opportunity to X-ray absorption fine structure (XAFS), a technique of directly speciate or determine the mode(s) of incorpora- XAS, uses photons of a specific energy (narrowly tuned tion of a particular element in a bulk sample. via a double-crystal monochromator to ~1 eV or less) to For this initial study we examined yttrium because it is couple with a specific electron energy level within the by far the most abundant YHREE (Table 1) in the Round element of interest and generate the emission of the cor- Top deposit at 221 g/t (or ppm) and provides an accessi- responding photoelectron. The emitted photoelectron ble K-edge for XAS. Yttrium is markedly similar in its wave backscatters when it encounters near-neighbor at- chemical and geochemical behavior to the HREEs, e.g., oms, with resultant positive or negative auto-interference, holmium and dysprosium. In 102 acid leaching experi- depending on the phase relation of the outgoing and ments (briefly described later), the correlation between % backscattered wave. Negative interference lowers the dysprosium extracted (percentage) and % yttrium extracted (percentage) was a remarkable 0.975, significant Table 2 Samples analyzed by XAS at better than the 99.9% confidence level. Thus yttrium Sample ID Grain size Leach time Y remaining/wt.% Temperature serves as a valid and robust proxy for the HREE suite at YBLK2TO4 2–4 mm N/A 100 N/A Round Top. Examination of the individual HREEs in YBLK0TO125 <0.125 mm N/A 100 N/A the rhyolite proved difficult due to interferences with ROCK1 Polished block N/A 100 N/A the X-ray emissions associated with the L-III edges of YHOT8 2–4 mm 3 d 40 70 ºC the HREEs from the K-edge excitations and emissions YS405D 2–4 mm 5 d 38 Room ~19 ºC of the major elements and transition metals in the rhyo- YS408322W 2–4 mm 2 weeks 26 Room ~19 ºC lite. The excitation energies of the K-edges of the YS4083211W 2–4 mm 11 weeks 23 Room ~19 ºC HREEs are beyond those normally available at syn- YS01258323D <0.125 mm 3 d 21 Room ~19 ºC chrotron sources.

PINGITORE Nicholas et al., Round Top Mountain rhyolite (Texas, USA), a massive, unique Y-bearing-fluorite- ... 93 overall photoelectron yield, which depends on the initial pounds (i.e., known to be present at Round Top) were and final photoelectron energy states. The phase relation obtained commercially to compare with the rhyolite depends on the photoelectron wavelength and the dis- samples. These are listed in Table 3. tance(s) between the origin atom and the near-neighbor atom(s). By incremental increase of the photoelectron Table 3 Model REE compounds for spectral comparison with samples energy beyond the absorption edge, additional energy is transferred to the photoelectron, increasing its kinetic en- Mineral Elemental formula Y/wt.% Collection locality Joseph Mine, Ojo Caliente ergy and therefore decreasing its wavelength. In this way, Y-bearing (Ca1–xY,HREEx )F2+x 1.7 District, Rio Arriba County, the atom of interest’s atomic near neighborhood is fluorite New Mexico, USA probed sequentially by photoelectrons of different wave- Y-bearing Innhavet, Drag, Nordland, (Ca1–xY,HREEx )F2+x 15.5 lengths, each generating a different auto-interference and fluorite Norway thus a different photoelectron yield, which is recorded Bastnäsite-Ce CeCO3F 0.02 Zagi Mine, Pakistan via the yield of characteristic X-rays emitted as the atom Bastnäsite-Ce CeCO3F 0.36 Unknown, Pakistan returns to its ground state. Thus a single experiment re- Xenotime-Y (Y,HREE)PO4 37.0 Bahia, Brazil sults in a spectrum of photoelectron yield versus X-ray Fluocerite-Ce (Ce,La)F3 1.1 Unknown, Brazil beam photon energy. This pattern can be used as a fin- Notes: 1. Sample analyses by electron microprobe gerprint, analogous to an X-ray diffraction pattern, for a specific compound or state (e.g., adsorption) by com- 3 Results and discussion parison with known materials, or can with the best data be solved for such structural and electronic parameters as 3.1 Spectra valence, number of neighbors, distances to them, and Fig. 3 presents extended X-ray absorption fine struc- their elemental identities. ture (EXAFS) spectra for samples and model compounds. XAS offers certain advantages over X-ray diffraction To assist in matching samples to model compounds, Fig. (XRD). First, because it excites a specific element, it can 3(a) shows the entire XAFS region as a single “finger- provide structural information at concentrations far lower print” with multiple ridges and valleys[11]. In both panels than XRD, in the ppm range rather than in the percentage we observed a strong pattern similarity among all the range. Second, effectiveness of XAS is not limited to re- Round Top samples. Some differences are the result of petitive (lattice) structures as is XRD, but can analyze data quality, i.e., one or more close minor peaks being sorbed atoms as well. An important advantage of XAS over clumped into a single peak or a humped peak due to electron microprobe analysis is that XAS in theory inter- lower resolution. Likewise, different specimens of each rogates every atom of, say, yttrium in a bulk, 3-dimensional of the model compounds are consistent within that sample (to the escape depth of the emitted characteristic model. X-rays), not just those in single grains specifically targeted It should be obvious that all the samples closely re- for analysis and residing at the surface of a sample. semble the spectra of the two Y-bearing fluorite model Details of XAS techniques and applications can be compounds. There is no resemblance at all to xeno- found in standard texts[7–10]. time-Y, and little to bastnäsite-Ce. Thus it appears that virtually all of the yttrium in these samples, which are 2.3 XAS experiments representative of the deposit, most possibly resides in the XAS experiments were performed at the Stanford fluorite-type crystal structure among the 4 phases exam- Synchrotron Radiation Lightsource (SSRL), a part of the ined in the present study. This speculation is reinforced Stanford Linear Accelerator (SLAC) located in Menlo by the samples that were leached in the laboratory in Park, CA, USA. Data were collected in fluorescence sulfuric acid, a process that removed much of the yttrium mode on beamline 7–3, using a 30-element germanium from the grains. Xenotime-Y is insoluble in sulfuric acid, detector, and on beamline 9–3 with a 100-element mono- and bastnäsite-Ce sparingly soluble or probably insoluble lith germanium detector. The synchrotron was operated in dilute sulfuric[12]. If any of the original yttrium resided in continuous fill mode, with a current ranging from 500 in either of these two phases or other insoluble minerals, to 495 mA (on 7–3), and at 100 mA with discrete fill (on the little yttrium remaining post-leaching would mostly 9–3). When appropriate, the beam was detuned 30%. be in those minerals and yield their respective spectra. Spectra for yttrium were collected at the K-edge, using This did not occur; the samples with decreasing percent- Soller slits and a Sr-3 filter to attenuate beam scatter. ages of remaining yttrium in Fig. 3 still retain only the Data were collected using XAS-Collect software and Y-bearing fluorite spectral signature. This indicates that processed with SIXPack and EXAFSPAK. perhaps 5% or less of the yttrium is not in the lattice of fluorite, i.e., 95% or more of the yttrium and by anal- 2.4 Model compounds ogy the HREEs are hosted in the Y-bearing fluorite. A total of 6 specimens of 4 relevant REE model com- Therefore the Round Top mineralogy is extremely

94 JOURNAL OF RARE EARTHS, Vol. 32, No. 1, Jan. 2014

Fig. 3 Spectra background-subtracted, normalized, k3-weighted, and plotted in k-space (a); radial distribution functions (RDF) derived from Fourier transforms of spectra in (a) (distances not corrected for phase shift) (b). No FT given for ROCK1 due to shorter k-space 3.5-cycle spectrum. YS4083211W and yttrofluorite NOR spectra smoothed to remove excess signal noise. favorable for extraction of the YHREEs by heap leach- The XAFS spectra of the 8 samples were entered into ing. the Principal Components Analysis program in the SIX- Minor differences between the spectra of the two Pack data analysis package. They yielded a single prin- Y-bearing fluorite model compounds can reasonably be cipal component, confirming mathematically that the attributed to the substitutional nature of this mineral. sample set is monomineralic. Target transformations to Therefore there is no specific set of elements in the each of the 6 model compounds indicated that the near-neighbor cation shells surrounding the yttrium at- Y-bearing fluorite from New Mexico was the best match oms. Given this fact, the nearly, but not perfectly, identi- to the samples. Thus the samples are possibly Y-bearing cal spectra of the two Y-bearing fluorites from separate fluorite. localities is not surprising. Further, minor differences in the sample spectra can be attributed to slightly different 3.3 Least-squares spectral fits materials in the different splits. Fig. 4 presents the fits of the Y-bearing fluorite from New Mexico to the two unleached samples and to two 3.2 Principal components analysis (PCA) leached samples, in which only 38% and 21% of the

PINGITORE Nicholas et al., Round Top Mountain rhyolite (Texas, USA), a massive, unique Y-bearing-fluorite- ... 95

equal importance is the solubility of the mineral, which dissolves rapidly in dilute sulfuric acid at room temperature. Sulfuric acid is inexpensive ($ 100–200 per ton; half that if generated in an on-site plant by combustion of sulfur) and universally available industrial commodity. This simplifies the liberation of YHREEs and suggests a heap leach model for ore processing. Because 90%–95% of the rhyolite rock volume is feldspars and quartz, minerals unreactive to dilute sulfuric acid, acid consumption is minimized. With the YHREEs hosted essentially exclusively in Y-bearing fluorite, we anticipate that very high heap leach recoveries can be achieved.

4.3 A unique deposit – or a new exploration target? Fig. 4 Fits of the spectrum of Y-bearing fluorite from New The Round Top rhyolite could be a unique deposit, Mexico (bold lines) to the spectra of the two unleached where Y-bearing fluorite is speculated as the main ore samples and to the spectra of two leached samples mineral of REEs. Nonetheless, given the current and fu-

ture projected economic value of YHREEs, it is wise to original yttrium remains. The fit is seen to be nearly consider that similar deposits could remain undiscovered identical in all cases. The differences are comparable to at the surface or, perhaps more likely, in the subsurface. the variation seen between the 2 different Y-bearing Because the economic demand for YHREEs is recent, fluorite standards themselves. Note that data quality suf- and this Y-bearing-fluorite-hosted YHREE deposit is fers in the leached samples due to their considerably neither well-known nor yet proved (i.e., in commercial smaller yttrium content. This reinforces our speculation production), there has been no exploration program(s) that Y-bearing fluorite is the only host mineral for yt- aimed to locate this target that we know of. Perhaps this trium in Round Top rhyolite. study will generate exploration interest for deposits of Y-bearing fluorite in the near future. 4 Significance of the Round Top deposit

4.1 Y-bearing fluorite: A unique mineralogy 5 Conclusions Fluorite is a common mineral. A part of calcium in X-ray absorption spectroscopy suggested that virtually fluorite can be replaced with rare earth elements to form all the yttrium, and therefore by proxy the HREEs, in the a solid solution series, (Ca1–xREEx)F2+x. The degree of Round Top Mountain rhyolite deposit was hosted in substitution is usually slight; it rarely is greater than 0.3 Y-bearing fluorite. The deposit was thus unique. Because apfu. The Y- or Ce-bearing varieties of fluorite have fluorite dissolved in dilute sulfuric acid at room tem- so-called variety names as “yttrofluorite,” “yttrocerite” perature, it appeared favorable that Y and HREEs could and/or “cerfluorite,” but they are not independent mineral be recovered at Round Top by inexpensive heap leaching. species. In November 2006 the Commission on New Since the yttrium, and by analogy, the HREEs were Minerals and Mineral Names of the International Min- hosted apparently exclusively in Y-bearing fluorite, heap eralogical Association discredited yttrofluorite as an in- leach recoveries likely would be very high. dependent mineral species. The term is, however, en- Acknowledgements: Authors thank the capable and helpful countered in pre-2006 literature, and remains in use as a staff at Standard Synchrotron Radiation Lightsource (SSRL), mineral variety name, e.g., the mindat.org website[13] . and especially Matthew Latimer, Erik Nelson, and Ryan Davis, The US Geological Survey’s (USGS) 2002 compen- for their assistance with our synchrotron experiments. Portions dium of hundreds of REE mines, deposits, and occur- of this research were carried out at the Stanford Synchrotron rences contains no reference to yttrofluorite or Y-bearing Radiation Lightsource, a Directorate of SLAC National Accel- fluorite deposits[14]. Only passing references to “yttro- erator Laboratory and an Office of Science User Facility oper- fluorite” or Y-bearing fluorite are found in such standard ated for the U.S. Department of Energy Office of Science by technical books as Rare Earth Minerals and Mineralogy Stanford University. The SSRL Structural Molecular Biology and Geology of Rare Earths in China[15,16]. Program was supported by the DOE Office of Biological and 4.2 Economic potential of Round Top Mountain Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (includ- Y-bearing fluorite may be a favorable economic REE ing P41GM103393) and the National Center for Research Re- resource because of both its yttrium content and its asso- sources (P41RR001209). The contents of this publication are ciated ability to preferentially incorporate the HREEs. Of

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